In coal seams characterized by"two hard"features,the determination of the limit size for protective coal pillars diverges from the standard approach for conventional working faces.This study,using the gob-side entry driving roadway of the 505 working face in Yushuling Coal Mine as a backdrop,first involved testing the mechanical properties of coal and rock samples and their compressive strength across various height-to-diameter ratios to establish peak strength curves.Subsequently,by analyzing the overlying rock structure model and constructing a mechanical model for the coal pillar roof in gob-side entry driving,the study identified the load stress experienced by coal pillars of varying widths.A numerical model for coal pillars with different aspect ratios was developed,yielding an ultimate strength verification formula for these pillars.Upon comparing the load stress and ultimate strength across different coal pillar widths,it was determined that a width of 4 meters for the coal pillar suffices to meet support requirements.Based on these findings,a support technology for gob-side entry driving is proposed.Field monitoring results indicate that with a coal pillar width of 4 meters,the overall deformation of the roadway is minimal,ensuring effective maintenance of roadway stability.The research provides a reference for determining the optimal width of small coal pillars in coal seams with"two hard"charac-teristics.
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